SeismologySubmarine cables could be repurposed as earthquake detectors

EARTH is observed as never before. Satellites track typhoons, monitor volcanic-ash plumes and catalogue the changing ways in which human beings use the land. The sort of high-quality imagery that, a couple of decades ago, was the preserve of spies in rich and powerful countries is now freely available to users of Google Maps.

But despite its name, most of Earth is covered in water, and it is much harder to monitor what goes on beneath the waves. In a paper just published in Science, Giuseppe Marra, of Britain’s National Physical Laboratory (NPL), proposes to shine a little light into the oceans by co-opting infrastructure built for an entirely different purpose. Dr Marra and his colleagues hope to use the planet’s 1m-kilometre network of undersea fibre-optic cables, which carry the internet from continent to continent (see map), as a giant submarine sensor.

Dr Marra is particularly interested in earthquakes. The dry bits of the planet are well-stocked with seismographs. The oceans are much less well covered, with only a handful of permanent sensors on the sea floor. This means that many small earthquakes go unrecorded because the vibrations they cause are too mild to be picked up by distant land-based sensors.

The genesis of the idea is a good example of the way in which advances in one field of science can lead to new developments in other, apparently unrelated fields. The NPL is Britain’s national metrology laboratory, devoted to the science of measurement. It is linked to other labs around Europe by fibre-optic cables that are used to synchronise the measurements of atomic clocks. Those cables often run beneath roads, and the vibration of traffic overhead introduces noise into the line that interferes with measurements, and must constantly be cancelled out.

Dr Marra proposes to use other sorts of noise to detect earthquakes. The idea is to shine a high-quality laser beam through one of the optical fibres in the cable. At the other end that fibre is connected to another in the same cable for the return journey, forming a loop. The seismic waves from a nearby earthquake will deform the cable minutely, leaving the returning light slightly out of phase with the light emitted by the laser. The discrepancies involved are tiny: on the order of millionths of a metre for a cable several thousand kilometres long. Measuring them requires equipment capable of discriminating between femtoseconds. A femtosecond is a millionth of a billionth of a second, which is roughly to a second what ten cents is to the GDP of the entire planet.

But Dr Marra’s bright idea works. In 2016, for instance, the NPL was able to spot a magnitude-six earthquake that had struck central Italy from the noise it produced in the fibre-optic cable, 79km long, which rather circuitously connects the NPL’s headquarters in London with a data centre in Reading. An underwater trial in 2017 used a 96km cable between Malta and Sicily. It detected a tremor of magnitude 3.4 that had an epicentre 89km from the cable’s nearest point.

One advantage of subsea cables is that they experience less noise. The Sicily-Malta cable had background noise levels a fifth to an eighth of those in cables on land. Dr Marra and his colleagues have not yet tested their system on a fully fledged ocean-crossing cable. But they hope that, when they do, things will be even quieter, helping them detect all sorts of seismic rumbles which go unheard today. That would be a boon to geologists. Dozens of cables cross the mid-Atlantic ridge, for instance. This is where the Eurasian and African tectonic plates drift away from those that carry North and South America, creating new crust in the process.

There may be other uses, too. In principle, the system can track any source of sound, from the migrations of animals such as dolphins and whales to the gas guns used in oil and gas exploration. If the communication cables that carry Earth’s data traffic can be used to work out what is going on in the 70% of the planet covered by water, then it really would count as a world-wide web.

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